1. Field of the Invention
The present invention relates generally to a method of making an aluminum-silicon alloy. Specifically, the present invention relates to a method of making an aluminum-silicon alloy in which a fine grain of silicon is formed.
2. Description of the Background Art
Generally, manufacturing of aluminum alloys for automotive members, such as piston for combustion or manifold for inlet and outlet, have been accomplished by die casting as die casting is convenient for mass production and space omission when manufacturing.
Conventionally, such aluminum alloys have been accomplished by casting Al-8Si alloy under conditions of high pressure to solidify the alloy, as is well known in the art. In this method, thermal conductivity between a die and the molten alloy are raised, that is, time for cooling the alloy is shorter, then grain size of silicon included in the alloy can be 20 to 30% finer compared with that formed by conventional gravity casting.
On the other hand, modification treatments of molten alloy by addition of a flux including Na, Sr, Sb, and/or Ca are also well known in the art in order to reduce the grain size of silicon.
Generally, the finess of grain in silicon greatly influences fatigue resistance of the alloy. For example, the tensile strength of aluminum-silicon alloy becomes larger as the eutectic silicon diameter therein becomes smaller.
However, both of the above-mentioned methods have limitations. When solidifying, a cooling time for a pressure cast alloy at wall thickness portions of an article formed of the alloy cannot be reduced easily compared to those at relatively thinner portions. On the other hand, using modification treatment, grain size of the eutectic silicon diameter cannot be controlled until the cooling speed of the alloy becomes relatively fast. Therefore, modification treatment is not sufficient for thicker portions of an article formed of the alloy. An alloy article formed by die casting may have a quite complicated shape, therefore, it is difficult to establish sufficiently fine silicon particles throughout the whole of the alloy article.
Thus, the grain size of silicon crystals becomes coarse and size and distribution of the silicon crystals varies depending on alloy thickness. That is, the alloy elements cannot be distributed homogeneously through the whole alloy. Therefore, when the alloy structure is stabilized by the well known solution heat treatment, mechanical characteristics of the alloy cannot be raised unless time for the solution heat treatment is prolonged.
Additionally, when an alumite coating is made on the desired portion of the alloy surface, thickness of the alumite coating cannot be made constant because various sizes of silicon crystals are distributed in the alloy. Further, the surface of the alumite coating becomes rough because the alloy surface becomes porous, thus mechanical strength of the alloy cannot be raised.